First, we must remember that life in the 1990s was not the same for everyone just as life today is not the same for everyone in the United States. Different people will have had different experiences of the 1990s because of their age, their sex, their social class, their race, and many other variables.
That said, the 1990s in the United States were generally a relatively prosperous and optimistic time. One reason for this was the position of the United States in the world. In late 1989, the Berlin Wall fell and by late 1991, the Soviet Union was officially dissolved. This meant that the US had won the Cold War and that Americans no longer felt threatened by the Soviets and their nuclear missiles. Since terrorism had not yet arisen as a major issue for most Americans, the world seemed safer for much of the 1990s.
Another reason for this was economic. Although there were worries during the early 1990s that Japan would come to dominate the world economically and that the US was falling behind, these were generally forgotten by mid-decade as the US economy started to boom. This was an era that was prosperous enough that the government had its first budget surpluses in decades. This, too, helped Americans to feel optimistic.
Finally, there were technological changes that made Americans feel as if life was progressing. The huge boom in personal computers and the beginnings of the internet revolution were chief among these. These technological changes, when combined with the changes in foreign policy and the economy, allowed many Americans to feel that life was getting better. Of course, not all Americans felt this way, but there is no way to accurately capture the experience of all people so we must generalize about the majority.